Process for making electrical conductors consisting of chromium oxide and titanium oxide



Patented Feb. 23, 1943 PROCESS :FOR

MAKING ELECTRICAL CON- DUCTORS CONSISTING OF CHROMIUM OXIDE AND TITANIUMOXIDE Eugene Winner and Norman R. Thielke, Niagara Falls, N. Y.,assignors to The Titanium Alloy Manufacturing Company, corporation ofMaine New York, N. Y., a

No Drawing. Application June 25, 1940, Serial No. 342,330

1 Claim.

This invention relates to new and useful compositions comprisingchromium titanates and methods of making the same. In electricalcircuits involving the use of high frequency currents, materials whichhave the combination of electrical properties of low or medianresistance, high power factor and fairly high dielectric constant, plusa large negative temperature coefiicient (that is, as the temperatureincreases the dielectric constant and power factor values rapidlyincrease) would be of considerable value in such circuits, particularlyif available in a. chemical form stable over a wide range oftemperature, pressure and chemical conditions, and if available in avariety of controlled particle sizes. Such materials would be useful asvolume controls for radio circuits where such volume control isdetermined by uniform variation of a standard resistance, as safetyvalves in high tension limiting voltages, as safety valves in circuitsinvolving changes in frequency where a limit is desired on the extent offrequency change, and as temperature limiting devices in high frequencyand/or high tension circuits. Furthermore, should such materials be of ahighly refractory nature the utility of the material is still furtherexpanded by the possibility of preparing a resistance heater fordevelopment of temperature between 2000 and 3000 F. by passing a highvoltage high frequency current through a suitably prepared and supportedbody.

In accordance with this invention, an entirely new group of compoundshas been discovered. These compounds are suitable for the uses listedabove, are highly refractory, and develop a large amount of heat when ahigh voltage high frequency current is passed through them. Thesecompounds are those which vary in composition from 2 moles of CrzOa permole of TiOz to 1 mole of Cl'zOs per 27 moles of TiOz, and areidentified as chromium titanates. A number of definite compounds mayexist in this series, some of which may have the following formulae:CriTiOs, Cl'ziTiOs, ClzTiaOis, ClzTimOzs, CrzTlzrOsv. On the other hand,certain of the members of this series may exist as mixtures of chromiumtitanates of different formulae, or mixtures of certain chromiumtitanates with TiOz or CrzOs. In fact mixtures outside the molecularlimits specified above have useful electrical properties.

In general, however, a composition varying from 6% to 79% C1'203 and 94%to 21% TiOz I is preferred. Each member of this series possesses therequired electrical properties, and finds utility in high frequencycircuits for the purposes mentioned above. All have relatively highfrequency circuits for low resistance, high power factor, highdielectric constant, high temperature coefiicients, and tend to act asenormous reservoirs of high frequency power as the temperature rises, inview of the fact that as the temperature increases the power factor anddielectric constant also increase. Since the amount of energy dissipated(in the form of heat) in the above described bodies is proportional tothe product of the dielectric constant and power factor, the applicationof such materials as safety valves in high frequency circuits and ashigh frequency resistance heaters (in view of their refractoriness) isobvious.

These compounds may be made in a variety of ways. For example, solutionsof the oxides in sulphuric acid may be mixed, evaporated to dryness andsuitably calcined, or a mixture of hydrates may be prepared byprecipitation from a solution of the oxides by the addition of alkalies,followed by calcination of the mixed hydrates. However, it is preferredto make these compounds by suitably calcining (in ordinary calcinationatmospheres, such as air, which are sufficiently oxidizing in nature toinsure that chromium is kept in the trivalent state and titanium in thequadrivalent state) an intimate mixture of the finely divided oxidesthemselves, such as at a temperature in excess of 2000 F. We have foundthat a fairly rapid and complete reaction is obtained at 2400 F. throughthe complete series, but we have also found that an enhancement of thedesirable electrical properties is obtained if the calcination iscarried out at 2600-2700 F. for a period about twice the time requiredto obtain complete reaction. A steel grey hard but friable sinter isobtained.

In preparing these compounds directly from the oxides, appropriatequantities of the oxides are mixed together, water is then added, andthe mixture is ground in a ball mill for a few hours to insurecompletely intimate mixture. The slip thus formed is dried and thenfired. The reacted product is then ground to coarse sizes by crushing inrolls. If a finely divided product is desired, the material is milledwith water in a ball mill to the desired size, dried and thendisintegrated. If a solid body is desired, the milled material may bepressed into the required shape and refired at an elevated temperature.

Examples of specific methods of practicing the invention are as follows:

EXAMPLE 1 grams of green chroslip is discharged and thoroughly dried andthen calcined for 8 hours at 2600' F., cooled, and then ground to -325mesh. Yield is 230 grams or approximately theoretical.

Emu: 2

To prepare CrzTiaOm: 152 grams of green chromium oxide is mixed with 640grams of pigment grade T102. batch is ground in a ball mill for 2 hours.The slip is discharged, thoroughly dried and calcined for 8 hours at2600 F., cooled and then ground to 325.mesh. Yield is 790 grams orapproximately theoretical.

Exam n- 3 To prepare CraTimOsvZ 152 grams of green chromium oxide ismixed with 2160 grams of pigment grade titanium oxide. 1500 cc. of wateris added and the batch is ground in a ball mill for 2 hours. The slip isdischarged, thoroughly dried and calcined for 8 hours at 2600 to 2700F., cooled and then ground to 325 mesh. Yield is 2300 grams orapproximately theoretical. The power factor and dielectric constant ofthis material at various temperatures are represented in the followingtable (measured at l megacycle) EXAMPLES 4-10 C1103 T10, 1 mole Cr Og+lmole Tig=152 g.: 80 2 moles Crg0i+ 3 moles TiO|=304 g. lmole CnOri-Zmoles T103=152 g. 1 mole Cr0;+ imoles Ti01=l52 g. 1 mole CnOs+ 8 molesTi0|=152 g. 1 mole (r;0;+l2 moles TiO|=l52 g. 1 mole Cheri-20 molesTiO1=152 g. Chromium oxide of ceramic grade and titanium oxide ofpigment grade were wet mixed in .a ball mill in the above proportions,dried, and then calcined as indicated in zircon crucibles. The calcinedresidues were milled through 325 mesh, dried, and then pressed intosquat cylinders approximately 1 inch in diameter and inch in thickness,These cylinders were then refired to 2650 for 3 hours, reaching thattemperature in 3 hours, to form a hard, dense mass.

The materials made according to Examples 4-10 were prepared for testingby applying a lead foil of 0.003 inch thickness to the fiat surfaces andattached by means of a thin film of petrolatum. The resistancecharacteristics of these sintered bodies werethen' determined by meansof a General Radio type 650A impedance bridge. Resistance was measuredfirst under a direct current of volts and then under an alternatingcurrent of 1000 cycles. The change of resistance with temperature ofthematerial of Example 10 was determined under direct current from roomtemperature to 150 C. The results of these tests are indicated in thefollowing tables:

550 cc. of water is added and the Tables D. C. re- A. C. re- Example No.sistivity at sistivity at 10 volts 1,000 cycles MeqJcm. lilac/cm.

2. 72 l. 18 2. 02 0 95 2. 1. 44 l. 59 0. 82 0. 91 0. 47 l. 56 0. 01 2.02 0. 91

TEMPERATURE D. C. RE QISTIVITY EXAMPLE 10 Temperature: Resistivity,Meg./ cm.

25 C 2.02- 70 C 1.69 C 1.57 95 C--- 1.45 C 0.95 C 0.76 C- 0.66

Preliminary measurements also indicate that at higher voltages, higherexciting frequencies, or higher temperatures or any combination of theseconditions, much lower resistivities are obtained.

EXAMPLE 11 900 grams of pigment grade T102, previously heated to 'atemperature of 1350 C. and maintained there for several hours, and 100grams of chromium titanate according'to Example 1, are thoroughly mixedby ball milling with water, dried and disintegrated. To this mixture 10%by weight of 5% gelatin solution is added and thoroughly incorporatedand the batch passed through a 20 mesh screen. 125 grams of the batch isplaced in the cavity of a steel mold 4 inches in diameter and thespecimen formed under a pressure of 1200 pounds per square inch. Thebody is then fired according to the following schedule: it is brought tothe peak temperature (2600 F.) at the rate of 40 F. per hour. It is heldtwelve hours'at the peak temperature, the temperature then decreased atthe rate of 50 F. per hour to 1200 F., then at the rate of 100 F. perhour from 1200 F. to 800 F., and the furnace then shut down. Theresistivity of this material was less than 3 megohms per centimeter,while the power factor was 8.8% and the dielectric constant 73.0.

In addition to the uses mentioned above, some of the chromium titanatesof the present invention possess properties which indicate possibilitiesin connection with the rectification of alternating electric currents.Furthermore, they may be utilized, in combination with titanium oxide,to enhance the photosensitive properties of the latter.

As many variations are possible within the scope of this invention, itis not intended to be limited except as defined by the appended claim.

We claim:

The method which comprises calcining a mixture consisting essentially of6 to 79% chromic oxide (CrzOs) and 94 to 21% titanium dioxide at atemperature in excess of 2000 F. for a period of time suflicient toinsure change of color of the mixture to a black color and underconditions sufllciently oxidizing to insure that the chromium is kept inthe trivalent state and the titanium in the quadrivalent state.

EUGENE WAINER. NORMAN R. THIELKE.

